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Mr Sorti said Mondadori decided not to reprint the book because of pressure from the Vatican.

"The book sold out immediately, and then people started to contact us to say that they wanted to buy a copy, but could not find it," said Mr Sorti. "We contacted Mondadori, and they said they would reprint it, but nothing ever happened."

The authors said the problems arose because they found documents in both the Vatican Secret Archive and the Italian State Archives which suggest that William of Orange was receiving huge sums of money from Rome.

In 1672, Louis had occupied the Netherlands. William led the resistance which drove out the French.

Although he was a fellow Roman Catholic, Louis was viewed as a threat by Rome and Pope Innocent XI secretly decided to back William.

William, who became King of Great Britain in 1689, has been revered in Ulster for centuries for his "Glorious Revolution", which drove James II, a Catholic, from the throne. He gives his name to the loyalist Orange Order.

Mr Sorti and Ms Monaldi said they found documents from a papal envoy discussing the "large sums" that William owed Innocent.

Corresponding documents from Innocent's family records show that the Holy See sent 150,000 scudi to William via an intermediary.

The sum, equivalent to more than £3.5 million today, equalled the Vatican's annual budget deficit.

"When we found the documents we had already started to write the book, but we decided to include the discovery as part of the storyline," said Ms Monaldi.

However, the revelation that Innocent was supporting a heretic and enemy of the Church embarrassed the Holy See and was seen as an impediment to Innocent's path to sainthood.

The Vatican began the canonisation process in 2002, at roughly the time the book was published. When the book came out, it was strongly criticized by leading Catholics and by the media.

Since then, Mr Sorti said he and his wife had been forced to leave Italy and settle in Vienna. "There has been real hysteria over it. People told us we were criminals," he said.

He added that a media black-out had descended in Italy. "There is a type of auto-censorship in Italy. When people know that a subject is risky, they avoid it at all costs," he said.

"Evidently we managed to upset someone senior in the Vatican, we do not know who, and after that no one would deal with us," he said.

A journalist from Rai, the state broadcaster, who asked not to be named, said that the book had become "a taboo".

A Vatican spokesman said: "It seems strange that they would accuse us of conspiracy. Certainly, if their historical novel has managed to sell 15,000 copies in Italy, they should be happy. This is a good figure for that type of book."

A Mondadori spokesman disputed the book's sales figures and added: "We are the Italian publishers of Dan Brown [the author of The Da Vinci Code], which was much harder to release, so I don't think we would have been troubled by the suggestions in this book."